The invention relates to a fluid catalytic cracking process for upgrading heavy petroleum stocks containing high molecular weight aromatics, such as resids and gas oils, to produce light and/or heavy distillate, while maintaining a high selectity of the process for isobutylene production in the C.sub.3 -C.sub.4 off gas production, during catalytic cracking operations.
The four C.sub.4 mono-olefins, 1-butene, cis-2-butene, trans-2-butene and 2-methylpropene are collectively called butylenes. The term isobutylene is by established usage interchangeable with the nomenclature 2-methylpropene, while the other three isomers are n-butenes. Often they are treated collectively because the four mono-olefins are obtained as mixtures, from natural gas and from petroleum refinery processes.
Isobutylene is a desirable reactant for the production of alkylate, an oligomer of petroleum refinery C.sub.3 -C.sub.4 off gases, which includes high octane gasoline components, and for the production of methyl-t-butyl ether, when isobutylene is reacted with methanol. A conventional process for separation of isobutylene from the other three components involves sulfuric acid extraction or selective adsorption, as the isomers cannot be separated by simple extraction. Acid extraction is cumbersome and includes as an undesirable aspect the oligomerization of the components themselves.
In known and conventional fluidized catalytic cracking processes, a relatively heavy hydrocarbon feedstock, e.g., a gas oil, admixed with a suitable cracking catalyst, e.g., a large pore crystalline silicate zeolite such as zeolite Y, to provide a fluidized suspension is cracked in an elongated reactor, or riser, at elevated temperature to provide a mixture of lighter hydrocarbon products. The gasiform reaction products and spent catalyst are discharged from the riser into a separator, e.g., a cyclone unit, located within the upper section of an enclosed stripping vessel, or stripper, with the reaction products being conveyed to a product recovery zone and the spent catalyst entering a dense catalyst bed within the lower section of the stripper. In order to remove entrained hydrocarbon product from the spent catalyst prior to conveying the latter to a catalyst regenerator unit, an inert stripping gas, e.g., steam, is passed through the catalyst where it desorbs such hydrocarbons conveying them to the product recovery zone. The fluidized catalyst is continuously circulated between the riser and the regenerator and serves to transfer heat from the latter to the former thereby supplying the thermal needs of the cracking reaction which is endothermic.
Particular examples of such catalytic cracking processes are disclosed in U.S. Pat. Nos. 3,617,497, 3,894,932, 4,309,279 and 4,368,114 (single risers) and U.S. Pat. Nos. 3,748,251, 3,849,291, 3,894,931, 3,894,933, 3,894,934, 3,894,935, 3,926,778, 3,928,172, 3,974,062 and 4,116,814 (multiple risers).
U.S. Pat. No. 3,894,932 describes a single riser fluid catalytic cracking operation in which a gas oil and a C.sub.3-4 -rich gaseous material is converted to aromatics and isobutane in the presence of a faujasite-type zeolite, e.g., zeolite Y.
U.S. Pat. No. 3,894,935 describes a dual riser fluid catalytic cracking process in which a gas oil is catalytically cracked in a first riser in the presence of a faujasite-type zeolite such as zeolite Y to provide gasoline boiling-range material and a C.sub.3-4 -rich hydrocarbon fraction including isobutylene which is converted in a second riser in the presence of hot regenerated catalyst or catalyst cascaded thereto from the first riser to provide aromatics, alkyl aromatics and low boiling gaseous material.
Several of the processes referred to above employ a mixed catalyst system with each component of the system possessing different catalytic properties and functions. For example, in the dual riser hydrocarbon conversion process described in U.S. Pat. No. 3,894,934, a heavy hydrocarbon first feed, e.g., a gas oil, is cracked principally as a result of contact with a large pore crystalline silicate zeolite cracking catalyst, e.g., zeolite Y, to provide lighter products. Spent catalyst is separated from the product stream and enters the dense fluid catalyst bed in the lower section of the stripping vessel. A C.sub.3-4 olefin-rich second feed, meanwhile, undergoes conversion to cyclic and/or alkylaromatic hydrocarbons in a second riser, principally as a result of contact with a shape selective medium pore crystalline silicate zeolite, e.g., zeolite ZSM-5. Spent catalyst recovered from the product stream of the second riser similarly enters the dense catalyst bed within the stripper vessel. U.S. Pat. No. 3,894,934 also features the optional introduction of a C.sub.3 -containing hydrocarbon third feed along with an aromatic-rich charge into the dense fluid bed of spent catalyst above the level of introduction of the stripping gas to promote the formation of alkyl aromatics therein. As desired, the third feed may be light gases obtained from a fluid cracking light ends recovery unit, virgin straight run naphtha, catalytically cracked naphtha, thermal naphtha, natural gas constituents, natural gasoline, reformates, a gas oil, or a residual oil of high coke-producing characteristics.
In this and other fluidized catalytic cracking operations employing mixtures of large and medium pore size crystalline silicate zeolite catalysts where catalyst separated from the product effluent is conveyed to a stripper and from there to a catalyst regenerating zone, regardless of the nature of the catalyst introduction at start-up, once steady-state operation has been achieved, the two types of catalyst will become fairly uniformly mixed and will circulate throughout the system at or about the same rate.